Biaxially oriented polyester film

ABSTRACT

Disclosed is a biaxially oriented polyester film composed mainly of a polymer blend comprising [A] a polyester having recurring units of the following general formula (I): ##STR1## wherein n is 2, 4 or 6 and R is at least one member selected from the group consisting of ##STR2## in which X is H or Cl and at least one X is Cl, and [B] a copolyester having units represented by the general formula (I) and units represented by the following general formula (II) and/or the following general formula (III): ##STR3## wherein R I , R II  and R III  stand for at least one member selected from the group consisting of 1,3-phenylene, 1,4-phenylene, 2,6-naphthalene, 2,7-naphthalene, ##STR4## and having a flow-initiating temperature not higher than 350° C. and a melt anisotropy-forming capacity, the molar ratio of the units represented by the general formula (II) and/or the general formula (III) being 0.5 to 18 mole % based on the total polymer blend. 
     One plane of the crystal of the polyester [A] is plane-oriented in the film surface, the in-plane orientation index is 0.75 to 0.95, and the crystal size in the crystal plane direction is 35 to 75 Å.

TECHNICAL FIELD

The present invention relates to a biaxially oriented polyester film.More particularly, the present invention relates to a biaxially orientedpolyester film having an excellent dimensional stability and a highelastic modulus, and suitable for use in the production of magnetictapes and flexible printed circuit boards.

BACKGROUND ART

A biaxially oriented polyester film is used as the base of a magnetictape or a flexible printed circuit board.

However, to obtain a labor-saving effect at the processing step in theproduction of a magnetic tape or a flexible printed circuit board, orwith an increase of the height of the application object, the demand fora film having an enhanced elastic modulus and a further improveddimensional stability is increasing. However, although the elasticmodulus of a biaxially oriented polyester film is increased by drawingin multiple stages in the film-forming process, the dimensionalstability is degraded by this multiple-stage drawing. Attempts toimprove the mechanical characteristics of the biaxially drawn polyesterfilm by making the film from a blend of a polymer having a rigid polymerchain, such as an aromatic polyester having a melt anisotropy-formingcapacity, with a polymer having a flexible polymer chain have been made.However, the mechanical characteristics are not improved because of aninsufficient dispersion of the rigid polymer, and because the rigidpolymer peels from the interface of the dispersed phase. Utilization canbe made only in a product obtained by drawing a fiber made of a blend ofa rigid chain polymer with a polyester to form voids in the interface ofthe dispersed phase, and thereby mat the film surface.

It is an object of the present invention to provide a polyester filmhaving a high elastic modulus, a low thermal shrinkage and an excellentdimensional stability, in which the defects of the conventionalbiaxially oriented polyester film are overcome by finely dispersing apolymer having a rigid molecule chain in a polyester.

DISCLOSURE OF THE INVENTION

According to the present invention, this object is attained by providinga biaxially oriented polyester film composed mainly of a polyblendcomprising [A] a polyester having recurring units of the followinggeneral formula (I): ##STR5## wherein n is 2, 4 or 6 and R is at leastone member selected from the group consisting of ##STR6## in which X isH or Cl and at least one X is Cl, and [B] a copolyester having unitsrepresented by the general formula (I) and units represented by thefollowing general formula (II) and/or the following general formula(III): ##STR7## wherein R^(I), R^(II) and R^(III) stand for at least onemember selected from the group consisting of 1,3-phenylene,1,4-phenylene, 2,6-naphthalene, 2,7-naphthalene, ##STR8## and having aflow-initiating temperature not higher than 350° C. and a meltanisotropy-forming capacity, the molar ratio of the units represented bythe general formula (II) and/or the general formula (III) being 0.5 to18 mole % based on the total polyblend, wherein one plane of the crystalof the polyester [A] is plane-oriented in the film surface, the in-planeorientation index is 0.75 to 0.95, and the crystal size in said crystalplane direction is 35 to 75 Å.

BEST MODE FOR CARRYING OUT THE INVENTION

In the polyester [A] constituting the film of the present invention,which has the recurring units represented by the following generalformula (I): ##STR9## n is an integer selected from the group consistingof 2, 4 and 6, but in view of the elastic modulus and thermal shrinkageof the film, a polymer in which n is 2 is especially preferred. In thegeneral formula (I), R is selected from the group consisting of1,4-phenylene, 2,6-naphthalene, ##STR10##

As specific examples of the structural units of the polyester [A], thefollowing units (a) through (e) can be mentioned, but in view of thefilm characteristics, polymers having structural units (a), (b) or (c)are preferred: ##STR11## Other components may be copolymerized in thepolyester [A], if the amount of the comonomer component is not largerthan 5 mole %. As the comonomer component, there can be mentioneddicarboxylic acids such as terephthalic acid, isophthalic acid,2,7-naphthalenedicarboxylic acid, andα,β-bis(phenoxy)ethane-4,4'-dicarboxylic acid, and dihydroxy compoundssuch as 1,4-cyclohexanediol and phenylhydroquinone.

The polyester [B] as the other constituent polymer of the film of thepresent invention comprises flexible units represented by the followinggeneral formula (I): ##STR12## wherein n is 2, 4 or 6, and R is##STR13## in which X is H or Cl and at least one X is Cl, and rigidunits represented by the following general formula (II) and/or thefollowing general formula (III) ##STR14## wherein R^(I), R^(II) andR^(III) stand for at least one member selected from the group consistingof 1,3-phenylene, 1,4-phenylene, 2,6-naphthalene, 2,7-naphthalene,##STR15## A polyester [B] in which parts of R^(I), R^(II) and R^(III)stand for at least one member selected from the group consisting of##STR16## is preferred, because the flow-initiating temperature islowered.

As specific examples of the rigid units represented by the generalformulae (II) and (III), the following units (a) through (o) can bementioned, and in view of the film characteristics, polymers comprisingat least one kind of unit selected from the units (a), (b), (d), (e),(f), (g), (i), (j), (k) and (m) are preferred: ##STR17##

The polyester [B] constituting the film of the present invention is apolyester having a flow-initiating temperature not higher than 350° C.and a melt anisotropy-forming capacity, and this polyester has achemical structure represented by the following general formula:##STR18##

The flow-initiating temperature of this polyester [B] is generallyinfluenced by the sum of the molar ratios of the units represented bythe general formulae (II) and (III), that is, (q+r). By the term"flow-initiating temperature" referred to herein is meant a lowesttemperature at which the polymer can flow. Where the copolymerizationratio (q+r) is high, the flow-initiating temperature of the copolyesteris often in agreement with the melting point of the polymer. When therandom degree of the copolymer sequence is increased, a definite meltingpoint cannot be confirmed by the thermal analysis process, but thepresence of the temperature at which the polymer begins to flow underheating is observed. In this case, this temperature is defined as theflow-initiating temperature.

Where the polyester [B] having a flow-initiating temperature not higherthan 350° C. is used, the film-forming property of the resulting polymerblend is very good and the characteristics of the formed film aregreatly improved. If the flow-initiating temperature is lower than 300°C., the film-forming property is especially improved. If theflow-initiating temperature exceeds 350° C., the melt-mixing temperatureor the film-forming temperature is elevated, and therefore, thermaldecomposition or deterioration of the polymer occurs. The lower limit ofthe flow-initiating temperature is not particularly critical, but thelower limit of the flow-initiating temperature is preferably 150° C.

The polyester [B] as one constituent polymer of the present inventionhas a melt anisotropy-forming capacity. In general, a polymer having amelt anisotropy-forming capacity comes to have a liquid crystalstructure showing an optical anisotropy when heated at a temperaturehigher than the flow-initiating temperature. However, even if thepolymer does not form a liquid crystal, when the polymer is insertedbetween two glass sheets and a relatively low shear rate of less than 10sec⁻¹ is imposed on the melt by fixing one glass sheet at a temperaturehigher than the flow temperature and sliding the other glass sheet, thepolymer takes the form of an optically anisotropic liquid showing a flowbirefringence. The optical anisotropy of the polymer melt can beobserved under crossed prisms by a polarizing microscope provided with aheating stage. Thus, a polymer showing an optical anisotropy in thestationary state or under a shear deformation of less than 10 sec⁻¹ whenheated at a temperature higher than the flow temperature is defined as apolymer "having a melt anisotropy-forming capacity".

As is seen from the foregoing description, the polyester [B] ischaracterized in that the polyester [B] has a melt anisotropy-formingcapacity and molecules of the polyester [B] are easily arranged andoriented in the stationary or flowing state. In order to retain thischaracteristic, preferably the sum (q+r) of the mole ratios of thecomponents represented by the general formulae (II) and (III) in themolecule is at least 40 mole %.

Examples of the polyester [B] having a melt anisotropy-forming capacity,to be used for the film of the present invention, which are preferablein view of the characteristics of the film, are as follows. ##STR19##

The copolymerization manner of the components (I), (II) and (III) of thepolyester [B] having a melt anisotropy-forming capacity according to thepresent invention may be either random copolymerization or blockcopolymerization, but blending of a random copolymer is preferred fromthe viewpoint of the production of the film because drawing for thebiaxial orientation is easily accomplished.

Other component may be copolymerized in the polyester, if the amount ofthis other component is not larger than 5 mole %. As this othercomponent, there can be mentioned dicarboxylic acids such asterephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid,α,β-bis(phenoxy)ethane-4,4'-dicarboxylic acid, and4,4'-diphenylcarboxylic acid, and dihydroxyl compounds such as1,4-cyclohexanedimethanol and phenylhydroquinone.

The film of the present invention is composed of a blend comprising thebase polyester [A] and the polyester [B]. The blending ratio of thepolyester [B] is selected so that the molar ratio of the sum of thecomponents represented by the general formulae (II) and (III) is 0.5 to18 mole % based on the total polyblend.

The molar ratio of the components of the general formulae (II) and (III)preferred from the viewpoint of the film-forming properties and the filmcharacteristics is 1 to 15 mole %, especially 2 to 10 mole %. If theblending ratio calculated as the molar ratio is lower than 0.5 mole %,little or no effect is attained by addition of the polyester [B] havinga liquid crystal-forming capacity, and it cannot be deemed that thecharacteristics are practically improved.

On the other hand, if the blending ratio exceeds 18 mole %, thedrawability of the polymer blend is considerably degraded and biaxialdrawing is extremely difficult, and such defects as a low elasticmodulus and a poor impact resistance appear in the obtained film.

The molar ratio F_(R) of the sum of the components represented by thegeneral formulae (II) and (III) is the ratio of the mole number X of therigid units of the total mole number Y of low-molecular-weight compoundsobtained by cutting all the ester linkages of the polymer blend, whichis defined by the following formula:

    F.sub.R =100×X/Y

Furthermore, X is represented by the formula of X=N_(m) +2X_(d), inwhich N_(m) stands for the mole number of the aromaticmonohydroxycarboxylic acid and X_(d) stands for the mole number of thearomatic dihydroxyl compound.

In general, F_(R) can be determined by hydrolyzing the polymer andquantitatively analyzing the decomposed components by using a gaschromatograph.

The preferred blending ratio X_(b) (% by weight) adopted when thepolyester [B] is blended in the polyester [A] depends on thecopolymerization ratio Mf (mole %) of the units of the formula (II)and/or the formula (III) in the polyester [B] and is expressed by thefollowing formula:

    1≦X.sub.b ≦-0.8M.sub.f +90

The blending ratio (% by weight) is defined by the following formula:##EQU1##

If the copolymerization ratio of the components (II) and (III) in thepolyester [B] is increased, the molecule chain becomes rigid and thecompatibility with the polyester [A] is changed. Therefore, the upperlimit of the preferred blending ratio is reduced.

The polyester [B] having a melt anisotropy-forming capacity ischaracterized in that, if the polyester [B] is deformed in the moltenstate, the polymer is elongated and the molecules are easily arrangedand oriented. This property is advantageous for improving the filmcharacteristics by controlling the dispersed form of the phase of thepolyester [B] appropriately. In order to maintain this characteristicproperty, preferably the copolymerization ratio Mf of the component (II)and/or the component (III) is at least 40 mole %.

The biaxially oriented polyester film of the present invention has ahigh elastic modulus, a low thermal shrinkage, and a high impactresistance, in combination. This film is characterized by the two-phasesystem comprising a dispersed phase and a continuous phase.

One plane of the crystal of the polyester [A] is plane-oriented in thefilm surface, and the in-plane orientation index is 0.75 to 0.95 and thecrystal size in the direction of this crystal plane is 35 to 75 Å.

The dispersed phase and continuous phase referred to herein are definedas follows. When two polymers having a poor compatibility aremelt-blended, the two polymers form discrete phases, and an island-seastructure is formed in which the phase of the polymer of a smalleramount is dispersed in the form of islands in the sea of the phase ofthe polymer of a larger amount. In this case, the portion of the islandsis called "the dispersed phase" and the portion of the sea is called"the continuous phase".

In the biaxially oriented polyester film of the present invention, sincethe polyester [B] is very finely phase-dispersed in the polyester [A],sometimes the phase of the polyester [B] cannot be observed by anordinary polarizing microscope under crossed prisms while heating thefilm at a temperature higher than the melting temperature of thepolymer. In this case, the interface between the phases can beconfirmed, for example, by the method in which the biaxially orientedfilm is cut in liquefied nitrogen and the cut face is carefully observedat 30,000 magnifications or more by a scanning electron microscope.

If the in-plane orientation index is smaller than 0.75, the impactresistance of the biaxially oriented film is reduced. If the in-planeorientation index exceeds 0.95, the film becomes brittle and a practicalnecessary impact resistance cannot be obtained.

If the crystal size is smaller than the lower limit value of theabove-mentioned range, the thermal stability of the film is reduced anda low thermal shrinkage cannot be obtained. If the crystal size exceedsthe upper limit value, the impact resistance is reduced and the filmcannot be put to practical use. Namely, in order to attain the object ofthe present invention, the in-plane orientation index and crystal sizeof the biaxially oriented film must be controlled within theabove-mentioned specific ranges.

Other polymers may be blended in any of the two polyesters [A] and [B]constituting the film of the present invention or a blend thereof in anamount not inhibiting the attainment of the object of the presentinvention, preferably in an amount smaller than 10% by weight.Furthermore, inorganic and/or organic additives such as an antioxidant,a heat stabilizer, a lubricant, a nucleating agent, and a surfaceprojection-forming agent may be added in customarily adopted amounts ifnecessary.

The process for the preparation of the film of the present inventionwill now be described. For the synthesis of the base polyester [A] ofthe film of the present invention, the following process, for example,can be adopted.

Polyethylene terephthalate (PET) used in the present invention isderived from ethylene glycol and terephthalic acid or derivativesthereof by polymerization according to known procedures. PET having areducing viscosity of at least 0.5 is used for forming a film having ahigh strength and a high elastic modulus.

A polyester containing 2,6-naphthalene-dicarboxylic acid is synthesizedby subjecting an alkylene glycol (having 2, 4 or 6 carbon atoms) and anester of 2,6-naphthalenedicarboxylic acid to ester exchange reaction inthe presence of a catalyst such as a calcium, magnesium or lithiumcompound at a temperature of 130° to 260° C., and carrying outpolycondensation at a temperature of 220° to 300° C. under a high vacuumin the presence of a catalyst such as an antimony or germanium compound.

A polyester containing structural units ofα,β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylic acid orα-(2-chlorophenoxy)-β-4,4'-dicarboxylic acid is synthesized in thefollowing manner. At first,α,β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylic acid or an esterderivative thereof is obtained, for example, by nucleus-chlorinatingp-hydroxybenzoic acid or an ester derivative thereof with chlorine gasto form 3-chloro-4-hydroxybenzoic acid or a derivative thereof andreacting this compound with an ethylene dihalide in the presence of analkali compound. Furthermore,α-(2-chlorophenoxy)-β-(phenoxy)ethane-4,4'-dicarboxylic acid or an esterderivative thereof is synthesized, for example, by reacting3-chloro-4-hydroxybenzoic acid synthesized according to theabove-mentioned process and p-hydroxybenzoic acid or ester derivativesthereof with an ethylene halide in the presence of an alkali compound.

The target polymer can be synthesized according to the directpolymerization process in which the above-mentioned twochlorine-containing dicarboxylic acids are subjected to ester-formingreaction with an alkylene glycol (having 2, 4 or 6 carbon atoms) in thepresence of a titanium or tin compound and polycondensation is carriedout at a temperature of 220° to 300° C. under a high vacuum in thepresence of a catalyst such as an antimony or germanium compound, or theester exchange process in which the above-mentioned chlorine-containingdicarboxylic acid ester derivatives are subjected to ester exchangereaction with an alkylene glycol (having 2, 4 or 6 carbon atoms) at atemperature of 130° to 260° C. in the presence of a catalyst such ascalcium, magnesium or lithium compound and polycondensation is carriedout at a temperature of 220° to 300° C. under a high vacuum in thepresence of an antimony or germanium compound.

Preferably, the melt viscosity of the so-obtained polyester [A] at ashear rate of 200 sec⁻¹ is 800 to 15,000 P, especially 1,100 to 8,500 P.Obviously, to obtain a polyester having a melt viscosity within thepreferred range, a process may be adopted in which the solid phasepolymerization is carried out after completion of the above-mentionedpolymerization.

The process for the synthesis of the polyester [B] having a liquidcrystal-forming capacity, which is used for the film of the presentinvention, will now be described.

An acid component is selected from ##STR20## and a dihydroxyl componentis selected from ##STR21## The dihydroxyl component is acetylated, andthe acetylated compound is blended with the acid component at a molarratio of 1/1. The blend is subjected to deacetylation reaction togetherwith the polyester [A] at 250° to 340° C. under a reduced pressure (10⁻¹to 10⁻² Torr), whereby the polyester [B] having a liquid crystal-formingcapacity is synthesized. Furthermore, deacetylation reaction of anacetylation product of a hydroxylcarboxylic acid such as ##STR22## or acomposition formed by adding an equimolar mixture of an acetylateddicarboxylic acid and an acetylated dihydroxyl compound to the aboveacetylation product is carried out at 250° to 340° C. under vacuum inthe presence of the polyester [A], and polycondensation is then carriedout to form a polymer having a liquid crystal-forming capacity. Toensure that the polyester [B] has a liquid crystal-forming capacity,preferably the mixing ratios of the foregoing various compounds areselected so that the ratio of the component (III) and/or the component(III) is 40 to 90 mole %. According to the kinds of carboxylic acid andhydroxyl compound used, the polymer composition is selected so that theflow-initiating temperature of the polyester [B] is not higher than 350°C.

The polyester [A] and the polyester [B] having a liquid crystal-formingcapacity, which have been synthesized according to the above-mentionedprocesses, are blended by the customary power- or pellet-blendingmethod. The blending ratio is selected so that the content of thecomponent represented by the general formula (II) and/or the componentrepresented by the general formula (III), present in the polyester [B],is 0.5 to 18 mole % based on the polymer blend.

The method for blending the polymers is not particularly critical.However, preferably the polyester [B] is dispersed in the polyester [A]as the main component of the film as finely as possible, and to attain afine dispersion of the polyester [B], use of a known static mixer or ascrew provided with a kneading zone is preferred. If blending is carriedout in the molten state for too long, the polymer composition is changedby ester exchange reaction and the expected film properties cannot beobtained.

The two polyesters are supplied to a known melt extruder after blendingor while they are being blended. The polymers supplied to the meltextruder are molten at 270° to 350° C., and the melt is extruded in theform of a sheet through a slit die and cooled and solidified by windingthe sheet around a casting drum controlled to a surface temperature of10° to 80° C. to form an undrawn film. In order to cool the sheetrapidly and uniformly, the electrostatic casting method is adopted. Thepolymer blend containing the polyester having a liquid crystal-formingcapacity is wound on the casting drum at a draft ratio of 3 to 30.

The polymer blend has a structure in which the polyester [B] having aliquid crystal-forming capacity is dispersed heterogeneously in thematrix of the polyester [A] as the main component. In this case, if theshape of the dispersed phase is that of a rod or wire having an axialratio larger than that of the sphere, the characteristics of the film ofthe present invention are especially excellent. Accordingly, the meltextruded from the melt extruder is cooled and solidified on the castingdrum while elongating the melt to draw the dispersed phase, andpreferably, the melt is wound at a draft ratio of 3 to 30.

Then, the so-obtained undrawn film is biaxially drawn. The knownsimultaneous biaxial drawing method and sequential biaxial drawingmethod can be adopted as the biaxial drawing method. In the sequentialbiaxial drawing method, in general, the film is first drawn in thelongitudinal direction and then drawn in the lateral direction. Thisdrawing order may be reversed. The biaxial drawing conditions differaccording to the properties of the polyesters constituting the film tobe drawn, the blending ratio, the drawing direction and the like, butpreferably the film is drawn at a temperature higher by 5° to 50° C.than the glass transition temperature of the main polyester constitutingthe continuous phase of the film at a drawing speed of 10³ to 10⁵ %/min,and further preferably, the draw ratio α in the longitudinal directionof the film and the draw ratio β in the lateral direction of the filmsatisfies the requirement of 12.5≦α² +β² ≦55.0 (α>2 and β>2).

The film drawn at the above-mentioned draw ratio is preferable forattaining the in-plane index and crystal size of the film specified inthe present invention.

The method in which the biaxially drawn film is drawn in at least onedirection again is effective for increasing the elastic modulus.

The drawn film is then heat-treated. The heat treatment is carried outon an oven or roll according to known procedures. The heat treatmentconditions for obtaining the film of the present invention differaccording to the kind of the polyester [A], but generally, the heattreatment is preferably carried out at 180° to 240° C. for 0.1 to 120seconds.

The film of the present invention may be subjected to a known coronadischarge treatment (in air, in nitrogen or in carbon dioxide gas). Inorder to impact adhesiveness, moisture resistance, heat sealability,lubricating property, and surface smoothness, the film may be used inthe form of a laminate with another polymer or when covered with anorganic and/or inorganic composition.

The copolyester having a liquid crystal-forming capacity in the film ofthe present invention is characterized in that the soft chain componentis copolymerized with the rigid component, and the copolyester ischaracteristic over a polyester composed solely of the rigid componentin that the flow-initiating temperature is low, the film-formingproperty is good, and the copolyester is finely dispersed in a polyestercomposed of the soft component. More specifically, at the step offorming the film of the present invention, little peeling of the layeror a formation of voids occurs in the dispersion interface, drawing ofthe film can be easily accomplished, the obtained biaxially orientedfilm has a high elastic modulus and an excellent dimensionalsubstability, and the biaxially oriented film is relatively transparentand has an excellent impact resistance.

The dispersed phase formed by the polyester having a liquidcrystal-forming capacity provides very roughened film surface to impartan easy slip characteristic to the film. Namely, the size of thedispersed phase, that is, the size of the convexities and concavities onthe film surface, can be controlled by changing the chemical structureof the polyester. Accordingly, fine convexities and concavities thatcannot be formed by a conventional inorganic particulate lubricant canbe formed on the surface, and a very slippery surface state can berealized.

Furthermore, since the rigid ester linkage of the polyester having aliquid crystal-forming capacity is subjected to little ester-exchangewith the ester linkage of the soft component, the film can beregenerated and used again.

The film of the present invention has a high elastic modulus and anexcellent dimensional stability in combination, and therefore, if thefilm of the present invention is formed into a magnetic tape, excellentelectric-magnetic conversion characteristics can be obtained. Moreover,the film of the present invention is valuable as a flexible printedcircuit substrate.

The film of the present invention can be applied to all uses to whichthe conventional biaxially oriented polyester films have been applied,but the film of the present invention is especially suitably for use asa base film of a magnetic tape used for a video or audio device, a basefilm for a magnetic disc, and a flexible printed circuit substrate. Thethickness of the film of the present invention is not particularlycirtical, but a film having a thickness of 1 to 15 μm, especially 4 to12 μm, is preferably for a magnetic tape that can be used for a longtime, and a film having a thickness of 50 to 150 μm is preferable for aflexible printed circuit board.

The characteristics referred to in the present invention are determinedand evaluated according to methods and standards described below.

(1) Flow-Initiating Temperature

The temperature at which the needle penetration thickness is at least90% of the thickness of the sample is measured according to thepenetration method using a thermal mechanical testing apparatus (TMA)supplied by Shinku Riko K.K., and this temperature is designated as theflow-initiating temperature. At the penetration test, a columnar quartzglass rod having a diameter of 1 mm is positioned vertically erect onthe polymer sheet and the temperature is elevated at a rate of 20°C./min while imposing a load of 1 g on the glass rod.

(2) Elastic Modulus and Specific Elastic Modulus

According to the method specified in JIS Z-1702, the elastic modulus ismeasured at a temperature of 25° C. and a relative humidity of 65% byusing an Instron type tensile tester. The elastic modulus of a biaxiallyoriented film is an arithmetic mean of the elastic modulus in thelongitudinal direction of the film and the elastic modulus in thelateral direction of the film. The specific elastic modulus of the blendfilm is defined by E/E₀ in which E₀ and E represent the elastic moduliof the PET film and blend film formed under the same conditions.

(3) Impact Resistance

According to the method specified in ASTM D-256, the Charpy impactstrength (unit: kg·cm/mm²) of the film is measured by a Charpy impacttester supplied by Toyo Seiki Seisakusho. An arithmetic mean value ofthe value obtained when the film is set horizontally between two fulcrain the longitudinal direction of the film and the value obtained whenthe film is set horizontally in the lateral direction of the film isadopted. When the Charpy impact strength is 20 or higher, it is judgedthat the impact resistance is good, and when the impact strength islower than 20, it is judged that the impact resistance is poor.

(4) Dimensional Stability (Thermal Shrinkage)

The sample film is cut into a specimen having a width of 10 mm and alength of 250 mm, and two mark lines are drawn with a distance of about200 mm. This distance (A mm) is precisely measured. A load of 3.0 g isimposed on the top end of the specimen, and in this state, the specimenis allowed to stand in a hot air oven at 180° C. for 10 minutes and thespecimen is then cooled to room temperature. The distance (B mm) betweenthe mark lines is measured. The value of 100×(A-B)/A is calculated foreach of the longitudinal direction and lateral direction of the film.The arithmetic mean value is designated as the thermal shrinkage of thefilm.

(5) Dimensional Stability Coefficient

The dimensional stability is defined by the formula of δ/(E-400) inwhich E stands for the elastic modulus of the film and δ stands for thethermal shrinkage of the film. When this value is smaller than 25×10⁻³,it is judged that the balance between the Young's modulus and thermalshrinkage is good, and when the value is 25×10⁻³ or larger, it is judgedthat this balance is not good.

(6) In-Plane Orientation Index

Strips (0.8 mm×10 mm) are cut out from the sample film so that the longsides are in agreement with the lateral direction of the film. Thesurfaces of these strips are piled together and the strips are combinedto form an X-ray diffraction sample having a thickness of 5 mm. Thesample is set so that the longitudinal direction of the film is inagreement with the direction of the X-rays. Then, the sample is tiltedby about 13° around the lateral direction of the film and X-rays aremade incident to obtain a diffraction peak. The incidence angle isminutely adjusted so that the incidence direction is in agreement withthe diffraction peak. While the sample is rotated around thelongitudinal direction of the film, the change of the diffractionintensity is observed. Supposing that the half-value width in the peakcurve showing the relationship between the rotation angle of the sampleand the diffraction intensity is Δ, the in-plane orientation index isgiven by the following formula: ##EQU2##

(7) Crystal Size

By using an X-ray diffraction apparatus, the diffraction peak isobserved by the reflection method while changing the lateral directionof the film and the incidence angle of X-rays. From the diffraction peakat about 13°, the crystal size D (Å) in the diffraction crystal planedirection of this peak is calculated according to the followingequation: ##EQU3## wherein B stands for the half-value width of thediffraction peak, b is 0.12°, λ stands for the wavelength of the K.sub.αray (1.5418 Å) of Cu, and θ stands for the diffraction angle of thepeak.

Embodiments of the present invention will now be described withreference to the following examples.

EXAMPLE 1

A pellet of PET (reducing viscosity: 0.66) prepared by polymerizationaccording to customary procedures was pulverized by a pulverizer toobtain a pulverization product.

Separately, 624 parts by weight of pulverized PET was mixed with 1,260parts by weight of p-acetoxybenzoic acid and polymerization was carriedout according to the deacetylation polymerization process [disclosed inJ. Polymer Sci., 14, 2043 (1976)] to obtain an ethyleneterephthalate/p-hydroxybenzoate copolymer (containing 70 mole % ofp-hydroxybenzoate). The polymer was pulverized by the pulverizer as PET.When the polymer was observed under crossed prisms at about 310° C. by apolarizing microscope provided with a heater, it was found that thepolymer had an optical anisotropy. The flow-initiating temperature ofthe polymer was lower than 350° C.

The pulverization products of PET and the ethyleneterephthalate/p-hydroxybenzoate copolymer were charged in a V-typeblender so that the amount added of the latter was 7% by weight (themolar ratio of the rigid component was 3.3 mole %), and the mixture wasblended for about 1 hour. The resulting blend and the pulverizationproduct of PET as a comparison were dried, independently supplied to anextruder having a screw diameter of 35 mm, and melt-extruded in the formof a sheet at 290° C. Each sheet was wound on a casting drum having asurface temperature of 20° C. at a winding speed corresponding to adraft ratio of 6 and cooled and solidified according to theelectrostatic casting method for forming a substantially unorientedundrawn film having a thickness of about 100 μm. The undrawn film waspre-heated at 80° C. and drawn in the longitudinal direction at adrawing temperature of 90° C. and a draw ratio of 3.0. Drawing waseffected by the difference of the peripheral speed between two sets ofrolls. The drawing speed was 50,000%/min. The monoaxially drawn film waspre-treated at 90° C. and drawn in the lateral direction at a drawingtemperature of 95° C. and a draw ratio of 3.2 by a tenter. The drawingspeed was 5,000%/min. The biaxially drawn film was heat-treated at 210°C. under a constant length for 15 seconds to obtain a film having athickness of about 11 μm. The specific elastic modulus of the film wasgood and 1.2, and the dimensional stability coefficient and impactresistance were good. The in-plane orientation index was 0.89 and thecrystal size was 52 Å.

The biaxially drawn film was cut into small pieces by a crusher, and thesmall pieces were compressed for form a pellet. The pellet was suppliedto an extruder and a biaxially drawn film was prepared again under thesame conditions as described above. The characteristics and structure ofthe film were not different from those of the originally prepared film.

EXAMPLES 2 THROUGH 4 AND COMPARATIVE EXAMPLES 1 AND 2

The ethylene terephthalate/p-hydroxybenzoate copolymer was blended in anamount of 5% by weight in Example 2, 10% by weight in Example 3 or 20%by weight in Example 4 into PET. Films were prepared under the followingconditions.

First, a substantially unoriented undrawn film having a thickness ofabout 110 μm, which was prepared in the same manner as described inExample 1, was simultaneously biaxially drawn at 85° C. and a draw ratioof 3.5 in each of the longitudinal and lateral directions by using afilm stretcher (supplied by T.M. Long Co.). The drawing speed was20,000%/sec. The biaxially drawn film was heat-treated at 210° C. undera constant length for 15 seconds to obtain a film having a thickness ofabout 10 μm. The physical properties of the so-prepared films are shownin Table 1. It is seen that each film had a good elastic modulus and anexcellent dimensional stability ccefficient and impact resistance.However, in Comparative Examples 1 and 2, in which the polymer having amelt anisotropy-forming capacity was blended in an amount of 0.5% byweight or 40% by weight into PET, the molar ratio of the rigid componentwas outside the range specified in the present invention, and as shownin Table 1, the film was poor in one or more of the elastic modulus,dimensional stability coefficient, and impact strength. In ComparativeExample 2, drawing could not be performed at the above-mentioned drawratio. Accordingly, drawing was carried out by changing each of the drawratios in the longitudinal and lateral directions of the film to 1.5.

EXAMPLES 5 AND 6 AND COMPARATIVE EXAMPLES 3 AND 4

Ethylene terephthalate/p-hydroxybenzoate copolymers having thecompositions shown in Examples 5 and 6 and Comparative Examples 3 and 4of Table 1 were synthesized in the same manner as described in Example 1except that the mixing ratio between PET and p-acetoxybenzoic acid waschanged as shown in Table 1. Only the polymer of Comparative Example 3had no melt anisotropy-forming capacity. Each polymer had a flowtemperature lower than 350° C.

The copolyester was blended in PET in an amount of 10% by weight inExample 5, 10% by weight in Example 6, 40% by weight in ComparativeExample 3 or 60% by weight in Comparative Example 4, and biaxially drawnfilms were prepared in the same manner as in Examples 2 and 3. Thephysical properties of these films are shown in Table 1. It is seenthat, if the composition of the copolyester and a meltanisotropy-forming capacity and the molar ratio of the rigid componentwas within the range specified in the present invention, thecharacteristics of the film were good. In contrast, even if the molarratio of the rigid component was within the range specified in thepresent invention, where the copolyester had no melt anisotropy-formingcapacity, the elastic modulus and impact resistance were poor. Note, inComparative Examples 3 and 4, drawing at the above-mentioned draw ratiowas impossible and, therefore, the draw ratio in each of thelongitudinal and lateral directions of the film was changed to 2.0.

COMPARATIVE EXAMPLE 5

The undrawn film obtained in Example 1 was drawn at a draw ratio of 4.0in the longitudinal direction at 85° C. while keeping the width constantby using a film stretcher. The drawing speed was 5,000%/min. Thewidth-fixed, monoaxially drawn film was heat-treated at 210° C. under aconstant length for 15 seconds. The obtained film had an in-planeorientation index of 0.59 and a crystal size of 50 Å, and the impactresistance was poor.

COMPARATIVE EXAMPLE 6

The biaxially drawn film obtained in Example 1 was heat-treated at 235°C. under a constant length of 100 hours. The obtained film had anin-plane orientation index of 0.97 and a crystal size of 70 Å. Thedimensional stability coefficient was good but the impact resistance waspoor.

EXAMPLE 7 AND COMPARATIVE EXAMPLES 7 AND 8

A mixture of 624 parts by weight of pulverized PET and 1,316 parts byweight of β-hydroxy-6-naphthoic acid was subjected to deacetylationpolymerization under the same conditions as described in Example 1 toform a copolyester having a melt anisotropy-forming capacity. The flowtemperature of the polymer was lower than 350° C.

The copolyester was blended with PET in an amount of 5% by weight inExample 7, 0.5% by weight in Comparative Example 7 or 40% by weight inComparative Example 8. Undrawn films were prepared in the same manner asdescribed in Example 1. Simultaneous biaxial drawing and heat treatmentwere carried out in the same manner as in Examples 2 through 4 to obtainbiaxially drawn films. Note, in Comparative Example 8, drawing could notbe performed, and therefore, the properties of the undrawn film wereevaluated. The physical properties of these films are shown in Table 1.It is seen that, if the molar ratio of the rigid component was withinthe range specified in the present invention, the physical propertieswere good, but if the molar ratio of the rigid component was outside therange specified in the present invention, the dimensional stabilitycoefficient or the impact resistance was poor.

                                      TABLE 1                                     __________________________________________________________________________    Composition of                                                                copolyester*       Molar ratio          Dimensional sta-                                  Construction                                                                         of rigid                                                                            In-plane                                                                            Crystal                                                                           Specific                                                                           bility coefficient                                                                       Impact                     Structural  ratio  component                                                                           orientation                                                                         size                                                                              elastic    Evalua-                                                                            resistance                 units       (molar ratio)                                                                        (mole %)                                                                            index (Å)                                                                           modulus                                                                            Value tion (evaluation)                                                                         Remarks             __________________________________________________________________________    Example 2                                                                           P/Q   P/Q = 30/70                                                                          2.4   0.89  53  1.2  13 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Example 3                                                                           P/Q   P/Q = 30/70                                                                          4.8   0.89  47  1.25  8 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Example 4                                                                           P/Q   P/Q = 30/70                                                                          9.7   0.89  51  1.25  5 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Com-  P/Q   P/Q = 30/70                                                                          0.024 0.90  53  1.0  53 × 10.sup.-3                                                                Poor Good   Outside             parative                                                  present             Example 1                                                 invention           Com-  P/Q   P/Q = 30/70                                                                          19.9  0.72  58  0.4  --    --   Poor   Outside             parative                                                  present             Example 2                                                 invention           Example 5                                                                           P/Q   P/Q = 20/80                                                                          5.7   0.87  55  1.2  12 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Example 6                                                                           P/Q   P/Q = 40/60                                                                          3.9   0.89  53  1.15 15 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Com-  P/Q   P/Q = 80/20                                                                          4.4   --    --  0.7  --    --   Poor   Outside             parative                                                  present             Example 3                                                 invention           Com-  P/Q   P/Q = 40/60                                                                          24.7  --    --  0.6  --    --   Poor   Outside             parative                                                  present             Example 4                                                 invention           Example 7                                                                           P/R   P/R = 30/70                                                                          1.9   0.85  53  1.2  11 × 10.sup.-3                                                                Good Good   Present                                                                       invention           Com-  P/R   P/R = 30/70                                                                          0.19  0.90  54  1.1  47 × 10.sup.-3                                                                Good Good   Outside             parative                                                  present             Example 7                                                 invention           Com-  P/R   P/R = 30/70                                                                          17.3  --    --       --    --   Poor   Outside             parative                                                  present             Example 8                                                 invention           __________________________________________________________________________     Note-                                                                         ##STR23##                                                                     ##STR24##                                                                     ##STR25##                                                                

EXAMPLE 8

A reaction vessel equipped with a rectifying column was charged with 85parts by weight (hereinafter referred to as "parts") of dimethylα,β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylate, 13.7 parts ofdimethyl α-(2-chlorophenoxy)-β-(phenoxy)ethane-4,4-dicarboxylate (molarratio: 85/15), 29.5 parts of ethylene glycol (ethylene glycol/totaldimethyl dicarboxylate molar ratio: 1.9/1), 0.075 part of calciumacetate, and 0.04 part of antimony trioxide, and the temperature wasgradually elevated with stirring over a period of 4 hours to a reactiontemperature of 140° to 245° C. to distill methanol in an amount of 99%(15.9 parts) based on the theoretical amount. Then, 0.02 part oftrimethyl phosphate was added to the reaction mixture and the esterexchange reaction product was transferred to a polymerization vessel.The temperature was elevated to 245° to 290° C. over a period of 1 hourand, simultaneously, the pressure was reduced to a high vacuum of lessthan 0.5 Torr over a period of 1 hour. The polycondensation was furtherconducted for 2 hours. The pressure was returned to atmospheric pressureby nitrogen, and the polymer was extruded in the form of a gut intowater under compression and the gut was cut to obtain a pellet. Thepellet was pulverized to obtain a pulverization product of a copolyesterhaving a melt viscosity of 1,900 poise (hereinafter referred to as"polymer I").

An ethylene terephthalate/p-hydroxybenzoate copolymer having a liquidcrystal-forming capacity (containing 70 mole % of p-hydroxybenzoate)(hereinafter referred to as "polymer II") was prepared by carrying outthe polymerization in the same manner as described in Example 1.

The polymer pellet was pulverized by a pulverizer to obtain apulverization product. When the polymer II was heated at about 290° C.and observed under crossed prisms by a polarizing microscope equippedwith a heater, it was found that the polymer II had an opticalanisotropy. The flow temperature was within the range specified in thepresent invention.

The pulverization products of the polymers I and II were charged in aV-type blender so that the molar ratio of the rigid component was about4 mole %, and were blended for about 1 hour (the blending ratio of thepolymer II was 5% by weight). The pulverization product blend and thepulverization product of the polymer I as a comparison were dried andwere separately melt-extruded in the form of a sheet at 290° C. throughan extruder having a screw diameter of 35 mm. According to theelectrostatic casting method, the melt sheet was wound on a casting drumhaving a surface temperature of 20° C. at a draft ratio of 6 and cooledand solidified to obtain a substantially unoriented undrawn film havinga thickness of about 110 μm. The undrawn film was pre-heated at 100° C.and drawn at a drawing temperature of 120° C. and a draw ratio of 3.2 inthe longitudinal direction. Drawing was effected by the difference ofthe peripheral speed between two sets of rolls, and the drawing speedwas 50,000%/min. The monoaxially drawn film was pre-heated at 110° C.and drawn at a drawing temperature of 120° C. and a draw ratio of 3.4 inthe lateral direction by using a tester. The drawing speed was5,000%/min. The biaxially drawn film was heat-treated at 230° C. under aconstant length for 15 seconds to obtain a film having a thickness of 10μm. The so-obtained film of the polymer I had an elastic modulus of 620kg/mm² and a thermal shrinkage of 4.4%. In contrast, the blendreinforced film had an elastic modulus of 700 kg/mm² and a thermalshrinkage of 1.0% and the film was proved to be excellent. Moreover, theimpact resistance was good, and the blend film had an in-planeorientation index of 0.93 and a crystal size of 6.1 Å.

EXAMPLE 9 AND COMPARATIVE EXAMPLES 9 AND 10

The polymers I and II obtained in Example 8 were blended so that theblending ratio of the polymer II was 10% by weight in Example 9, 0.5% byweight in Comparative Example 9 or 30% by weight in Comparative Example10, and biaxially drawn films were obtained in the same manner asdescribed in Example 8. The physical properties of the obtained filmsare shown in Table 2. It is seen that, if the molar ratio of the rigidcomponent was within the range specified in the present invention, theelastic modulus was high, the thermal shrinkage was low, and the impactresistance was good. If the molar ratio of the rigid component wasoutside the range specified in the present invention, although theelastic modulus was low, the thermal shrinkage was relatively high orthe impact resistance was poor. Note, in Comparative Example 10, drawingcould not be performed under the conditions described in Example 8, andtherefore, the film was prepared by changing the draw ratio in thelongitudinal direction to 1.5 and the draw ratio in the lateraldirection to 1.4.

EXAMPLE 10 AND COMPARATIVE EXAMPLES 11 AND 12

A copolymer of polyphenylhydroquinone terephthalate and polyethyleneterephthalate (hereinafter referred to as "polymer III") was prepared bycarrying out deacetylation polymerization of acetylatedphenylhydroquinone and terephthalic acid in the presence of polyethyleneterephthalate (according to the process disclosed in U.S. Pat. No.4,159,365). The amount added of polyethylene terephthalate was adjustedso that the copolymerization ratio of the ethylene terephthalate unitswas 5 mole %. The obtained polymer pellet was pulverized by apulverizer. When the polymer was heated at about 300° C. and observedunder crossed prisms by a polarizing microscope, the polymer showed anoptical anisotropy. The flow temperature of the polymer was within therange specified in the present invention.

The pulverization products of the polymer I obtained in Example 8 andthe polymer III were blended so that the blending ratio of the polymerIII was 5% by weight in Example 10, 0.5% by weight in ComparativeExample 11 or 25% by weight in Comparative Example 12, and biaxiallydrawn films were prepared under the same conditions are described inExample 8. The physical properties of the obtained films are shown inTable 1. It is seen that, if the molar ratio of the rigid component waswithin the range specified in the present invention, the elastic moduluswas high, the thermal shrinkage was low, and the impact resistance wasgood. In contrast, if the molar ratio of the rigid component was outsidethe range of the present invention, the reinforcing effect by blendingwas not attained, drawing of the film was impossible (ComparativeExample 12) or the impact resistance was poor.

COMPARATIVE EXAMPLE 13

The undrawn film obtained in Example 8 was drawn at 120° C. and a drawratio of 4.0 in the longitudinal direction while fixing the width of thefilm by using a film stretcher (supplied by T.M. Long Co.). The drawingspeed was 5,000%/min. The width-fixed, monoaxially drawn film washeat-treated at 230° C. under a constant length for 15 seconds. Thein-plane orientation index of the obtained film was 0.60 and the crystalsize was 60 Å. The impact resistance was poor.

COMPARATIVE EXAMPLE 14

The biaxially drawn film obtained in Example 10 was heat-treated at 240°C. under a constant length for 100 hours. The in-plane orientation indexof the obtained film was 0.97 and the crystal size was 79 Å. The plasticmodulus and dimensional stability of the film were good, but the filmwas brittle and the impact resistance was poor.

EXAMPLE 11 AND COMPARATIVE EXAMPLES 15 AND 16

A blend of 244 parts of methyl 2,6-naphthalenedicarboxylate and 62 partsof ethylene glycol was polycondensed under the same conditions asdescribed in Example 8 to obtain poly(ethylene 2,6-naphthalate)(hereinafter referred to as "polymer IV"). The polymer IV was mixed withthe polymer III obtained in Example 3 so that the amount of the polymerIII was 5% by weight in Example 11, 0.5% by weight in ComparativeExample 15 or 25% by weight in Comparative Example 16. A substantiallyunoriented undrawn film having a thickness of about 120 μm was preparedin the same manner as described in Example 8. The film wassimultaneously biaxially drawn at 145° C. and a draw ratio of 3.3 byusing a film stretcher (supplied by T.M. Long Co.). The drawing speedwas 10,000%/min. The simultaneously biaxially drawn film washeat-treated at 230° C. under a constant length for 15 seconds to obtaina film having a thickness of about 10 μm. The physical properties of theso-obtained films are shown in Table 2. It is seen that if the molarratio of the rigid component was within the range specified in thepresent invention, the elastic modulus was high, the thermal shrinkagewas low, and the impact resistance was good. The film of the polymer IValone prepared as a comparison had an elastic modulus of 590 kg/mm² anda thermal shrinkage of 20%.

In contrast, if the molar ratio of the rigid component was outside therange specified in the present invention, drawing was impossible(Comparative Example 16) or the reinforcing effect by blending was notsubstantially manifested.

                                      TABLE 2                                     __________________________________________________________________________           Polymer having liquid                                                                        Molar ratio                                                    crystal-forming capacity                                                                     of rigid                                                                            Elastic                                                                             Thermal                                                                             Impact In-plane                                                                            Crystal                         Structural                                                                          Construction ratio                                                                     component                                                                           modulus                                                                             shrinkage                                                                           resistance                                                                           orientation                                                                         size                            units (molar ratio)                                                                          (mole %)                                                                            (kg/mm.sup.2)                                                                       (%)   (evaluation)                                                                         index (Å)                                                                            Remarks             __________________________________________________________________________    Example 9                                                                            P/Q   P/Q = 30/70                                                                            7.5   744   0.5   Good   0.90  66   Present                                                                       invention           Comparative                                                                          P/Q   P/Q = 30/70                                                                            0.39  610   4.1   Good   0.93  67   Outside             Example 9                                                 present                                                                       invention           Comparative                                                                          P/Q   P/Q = 30/70                                                                            20.7  300   0.3   Poor   0.80  70   Outside             Example 10                                                present                                                                       invention           Example 10                                                                           P/R   P/R = 5/95                                                                             5.8   700   0.7   Good   0.90  64   Present                                                                       invention           Comparative                                                                          P/R   P/R = 5/95                                                                             0.31  620   4.0   Good   0.90  69   Outside             Example 11                                                present                                                                       invention           Comparative                                                                          P/R   P/R = 5/95                                                                             27.5  --    --    Poor   --    --   Outside             Example 12                                                present                                                                       invention           Example 11                                                                           P/R   P/R = 5/95                                                                             3.7   640   0.5   Good   0.92  60   Present                                                                       invention           Comparative                                                                          P/R   P/R = 5/95                                                                             0.37  550   1.8   Good   0.94  63   Outside             Example 15                                                present                                                                       invention           Comparative                                                                          P/R   P/R = 5/95                                                                             19.3  --    --    Poor   --    --   Outside             Example 16                                                present                                                                       invention           __________________________________________________________________________     Note                                                                          ##STR26##                                                                     ##STR27##                                                                     ##STR28##                                                                

CAPABILITY OF EXPLOITATION IN INDUSTRY

The biaxially oriented polyester film of the present invention has ahigh elastic modulus, a low thermal shrinkage and an excellentdimensional stability. Accordingly, this film is valuable for theproduction of, for example, a magnetic tape and a flexible printedcircuit substrate.

We claim:
 1. A biaxially oriented polyester film composed mainly of apolymer blend comprising [A] a polyester having recurring units of thefollowing general formula (I); ##STR29## wherein n is 2, 4 or 6 and R isat least one member selected from the group consisting of ##STR30## inwhich X is H or Cl and at least one X is Cl, and [B] a copolyesterhaving units represented by the general formula (I) and unitsrepresented by the following general formula (II) and/or the followinggeneral formula (III): ##STR31## wherein R^(I), R^(II) and R^(III) standfor at least one member selected from the group consisting of1,3-phenylene, 1,4-phenylene, 2,6-naphthalene, 2,7-naphthalene,##STR32## and having a flow-initiating temperature not higher than 350°C. and a melt anisotropy-forming capacity, the molar ratio of the unitsrepresented by the general formula (II) and/or the general formula (III)being 0.5 to 18 mole % based on the total polyblend, wherein one planeof the crystal of the polyester [A] is plane-oriented in the filmsurface, the in-plane orientation index is 0.75 to 0.95, and the crystalsize in said crystal plate direction is 35 to 75 Å.
 2. A polyester filmas set forth in claim 1, wherein the molar ratio of the unitsrepresented by the general formulae (II) and (III) is 1 to 15 mole %based on the total polymer blend.
 3. A polyester film as set forth inclaim 1, wherein the molar ratio of the recurring units represented bythe general formulae (II) and (III) is 2 to 10 mole % based on the totalpolymer blend.
 4. A polyester film as set forth in claim 1, wherein inthe recurring units constituting the polyester [A], which arerepresented by the general formula (I), n is
 2. 5. A polyester film asset forth in claim 1, wherein the recurring units constituting thepolyester [A], which are represented by the general formula (I), arerepresented by the following formula: ##STR33##
 6. A polyester film asset forth in claim 1, wherein the recurring units constituting thepolyester [A], which are represented by the general formula (I), arerepresented by the following formula: ##STR34##
 7. A polyester film asset forth in claim 1, wherein the recurring units constituting thepolyester [A], which are represented by the general formula (I), arerepresented by the following formula: ##STR35##
 8. A polyester film asset forth in claim 1, wherein in the recurring units constituting thepolyester [B], which are represented by the general formulae (II) and(III), at least one of R^(I), R^(II) and R^(III) is selected from thegroup consisting of ##STR36##
 9. A polyester film as set forth in claim1, wherein the flow-initiating temperature of the polyester B is 150° to300° C.
 10. A polyester film as set forth in claim 1, wherein the sum ofthe molar ratios r and q of the components represented by the generalformulae (II) and (III) in the polyester [B] is at least 40 mole %. 11.A polyester film as set forth in claim 1, wherein the proportion X_(b)(% by weight) of the polyester [B] in the blend of the polyester [A] andthe polyester [B] satisfies the requirement represented by the followingformula:

    1≦X.sub.b ≦-0.8M.sub.f +90

wherein Mf stands for the copolymerization ratio (mole %) of thecomponents represented by the general formulae (II) and (III) in thepolyester [B].